Electrowetting on dielectric (EWOD) is a well-known technique for manipulating droplets of fluid by the application of an electric field. Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in an active matrix array incorporating transistors, for example by using thin film transistors (TFTs). It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 10, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of array element electrodes 12 (e.g., 12A and 12B in FIG. 1) are realized. The electrode of a given array element may be termed the element electrode 12. A liquid droplet 14, including a polar material (which is commonly also aqueous and/or ionic), is constrained in a plane between the lower substrate 10 and a top substrate 16. A suitable gap or channel between the two substrates may be realized by means of a spacer 18, and a nonpolar surround fluid 20 (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet 14. The function of the oil is to reduce the surface tension at the surfaces of the polar droplets, and to increase the electro-wetting force, which ultimately leads to the ability to create small droplets and to move them quickly. It is usually beneficial, therefore, for the oil to be present within the channel of the device before any polar fluids are introduced therein.
An insulator layer 22 disposed upon the lower substrate 10 separates the conductive element electrodes 12A, 12B from a first hydrophobic coating 24 upon which the liquid droplet 14 sits with a contact angle 26 represented by θ. The hydrophobic coating is formed from a hydrophobic material (commonly, but not necessarily, a fluoropolymer). On the top substrate 16 is a second hydrophobic coating 28 with which the liquid droplet 14 may come into contact. Interposed between the top substrate 16 and the second hydrophobic coating 28 is a reference electrode 30.
The contact angle θ is defined as shown in FIG. 1, and is determined by the balancing of the surface tension components between the solid-to liquid (γSL), the liquid-to non-polar surrounding fluid (γLG) and the solid to non-polar surrounding fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by:
                              cos          ⁢                                          ⁢          θ                =                                            γ              SG                        -                          γ              SL                                            γ            LG                                              (                  equation          ⁢                                          ⁢          1                )            
In operation, voltages termed the EW drive voltages, (e.g. VT, V0 and V00 in FIG. 1) may be externally applied to different electrodes (e.g. reference electrode 30, element electrodes 12, 12A and 12B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic coating 24. By arranging for different EW drive voltages (e.g. V0 and V00) to be applied to different element electrodes (e.g. 12A and 12B), the liquid droplet 14 may be moved in the lateral plane between the two substrates 10 and 16.
Example configurations and operation of EWOD devices are described in the following. U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions. U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials. U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based thin film electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electrowetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based thin film electronics to control an EWOD array, namely:                Electronic driver circuits can be integrated onto the lower substrate 10.        TFT-based thin film electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost.        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require electro-wetting voltages in excess of 20V to be applied.        
FIG. 2 is a drawing depicting additional details of an exemplary AM-EWOD device 36 in schematic perspective, which may incorporate the layered structures in FIG. 1. The AM-EWOD device 36 has a lower substrate 44 with thin film electronics 46 disposed upon the lower substrate 44, and a reference electrode (comparable to reference electrode 30 above) is incorporated into an upper substrate 54. The electrode configuration may be reversed, with the thin film electronics being incorporated into the upper substrate and the reference electrode being incorporated into the lower substrate. The thin film electronics 46 are arranged to drive array element electrodes 48. A plurality of array element electrodes 48 are arranged in an electrode or element array 50, having X by Y array elements where X and Y may be any integer. A liquid droplet 52 which may include any polar liquid and which typically may be aqueous, is enclosed between the lower substrate 44 and the upper substrate 54 separated by a spacer 56, although it will be appreciated that multiple liquid droplets 52 can be present.
As described above with respect to the representative EWOD structure, the EWOD channel or gap defined by the two substrates initially is filled with the nonpolar fluid (oil). The liquid droplets 14/52 including a polar material, i.e., the droplets to be manipulated by operation of the EWOD device, must be inputted from an external “reservoir” of fluid into the EWOD channel or gap. The external reservoir may for example be a pipette, or may be a structure incorporated into the plastic housing of the device. As the fluid from the reservoir for the droplets is inputted, oil gets displaced and is removed from the EWOD channel.
Different mechanisms have been devised for the inputting or loading of fluid into such devices. For example, U.S. Pat. No. 8,686,344 (Sudarsan et al., issued Apr. 1, 2014) describes a method of fluid loading utilizing patterning of the hydrophobic layers disposed upon the device surfaces. WO 2015/023747 (Yi et al., published Feb. 19, 2016) and US 2016/0016170 (Lay et al., published Jan. 21, 2016) both describe an EWOD cartridge assembly including upper and lower EWOD substrates and a plastic part which functions as a pipette guide.
Achieving entry of the polar fluid into the EWOD channel is non-trivial because the internal surfaces of the EWOD device are hydrophobic. Additionally, the direction of travel of the fluids once in the EWOD channel must be controlled, for example such that different fluids input through different and adjacent ports do not accidentally combine together or mix.
A conventional method of achieving controlled fluid entry is to create an upper substrate with apertures (holes) drilled or otherwise incorporated into the upper substrate. Apertures in the upper substrate provide for convenient fluid input, but of course require the thin film electronics to be on the bottom substrate. The apertures define fluid input ports and a fluid path from the exterior of the EWOD device into the EWOD channel directly above the electrowetting array. Employing apertures in the upper substrate, however, may be difficult and expensive to manufacture, particularly because the preferred material of the upper and lower substrates is glass. Glass is commonly the preferred material because glass is compatible with common LCD manufacturing technologies. It furthermore is advantageous to make the EWOD device with the same materials for the upper and lower substrates to achieve a high precision in the EWOD channel gap, and to minimize deleterious effects of working with different materials of different thermal expansion coefficients for usages in which the device is to be heated. In normal usage, the number of distinct polar fluid entry points is determined by the number of apertures within the upper substrate. As the number of apertures in the upper substrate is increased, both the cost of production and the fragility of the upper substrate goes up.
An alternative approach is to use a side loading configuration that does not require apertures formed in the upper substrate (e.g., the upper substrate is simply rectangular). Fluids that are to enter the EWOD channel are inputted through a side of the EWOD channel between the two substrates, rather than through apertures in the upper substrate. By using an upper substrate with no apertures, the cost and mechanical strength of the upper substrate is completely independent of the number of polar fluid entry points that are required, potentially enabling a higher density of fluid entry points than can be incorporated into the EWOD device when the upper substrate has apertures. It has been difficult, however, to achieve precise control of polar fluid input with current side loading configurations.
GB 2542372 (Walton et al., published Mar. 22, 2017) is another design by the current inventors. Such disclosure describes a method of fluid loading and discloses a spacer design for side-loading of fluid into the EWOD channel. This simple side loading, however, does not incorporate any particular components for ensuring that polar fluid input from outside of the EWOD device is specifically able to enter the EWOD channel.